1. Field of the Invention
The present invention relates to an image pickup apparatus capable of using a flash apparatus.
2. Description of the Related Art
In an image pickup apparatus such as an electronic camera, it is required to use a flash apparatus (flash) when a picture of a subject is taken in a low ambient light environment or in a backlighted situation.
It is also required to adjust the white balance in accordance with the color temperature of a subject. However, in the case where artificial lighting is employed, color rendering is poor and thus correction of only the white balance is not sufficient to obtain good color balance in many cases. In view of the above, various techniques have been proposed for correcting a hue or chroma saturation depending on the white balance. Some of those conventional techniques are described below with reference to the drawings.
FIG. 6 is a block diagram illustrating an image sensing system used in a conventional electronic camera. As shown, the image sensing system include a lens 201, an iris diaphragm 202, an image sensing device 203, a luminance signal generating circuit 204 for generating a luminance signal in accordance with the output of the image sensing device, a luminance signal processing circuit 205 for performing processing such as gamma processing and edge enhancement, a color separation circuit 206 for generating color signals associated with three colors R, G, and B, a white balance amplifier 207 for controlling the white balance by adjusting the gains associated with the R-signal and B-signal, a color difference matrix circuit 208 for generating color difference signals R-Y and B-Y from the R-, G- and B-signals output from the white balance amplifier 207, a linear matrix circuit 209 for adjusting the hue, a color difference gain control circuit 210 for adjusting the color saturation by controlling the gains of the color difference signals R-Y and B-Y, an encoder 211 for generating a video signal from the luminance signal and the color difference signals, a white balance control circuit 212 for extracting color temperature information from the color difference signals and controlling the gain of the white balance amplifier 207, a hue correcting circuit 213 for correcting the hue and the chroma saturation based on the white balance information supplied from the white balance control circuit 212, a flash apparatus 214, and a flash control circuit 215 for controlling the timing and the amount of light generated by the flash apparatus 214.
Light incident on the lens 201 is passed through the iris diaphragm 202 and adjusted as to control the amount of exposure. The light then falls on the image sensing device 203. The image sensing device 203 converts the received light to a corresponding electric charge and accumulates it. The image signal output from the image sensing device 203 is separated by the luminance signal generating circuit 204 into a luminance signal Y and a color signal. The resultant luminance signal Y is applied to the luminance signal processing circuit 205 and subjected to processing such as gamma processing and edge enhancement.
On the other hand the color signal is applied to the color separation circuit 206 and separated into three color signals R, G, and B. The white balance amplifier 207 makes a white balance adjustment on the R-, G-, and B-signals and also performs gamma processing on them. The resultant R-, G-, and B-signals are applied to the color difference matrix circuit 208 and converted into color difference signals R-Y and B-Y. The resultant color difference signals R-Y and B-Y are applied to the linear matrix circuit 209 and adjusted in terms of the hue.
The signals output from the linear matrix circuit 209 are applied to the color difference gain control circuit 210 and the gains of the respective color difference signals R-Y and B-Y are adjusted. The resultant signals output from the color difference gain control circuit 210 are applied to the encoder 211 and encoded into a video signal. The signals output from the color difference gain control circuit 210 are also supplied to the white balance control circuit 212. The white balance control circuit 212 detects the color temperature from the color difference signals. The white balance control circuit 212 then calculates the gain of the white balance amplifier 207 in accordance with the detected color temperature and controls the white balance.
The flash control circuit 215 controls the flash apparatus 214 such that light is generated by the flash apparatus 214 during a charge accumulation period of the image sensing device 203, wherein the amount of light is controlled in accordance with the luminance signal Y.
The color temperature of a subject varies depending on the type of a light source. When artificial lighting is employed, color rendering is poor and thus correction of only the white balance is not sufficient to obtain good color balance in many cases. Therefore it is required to correct also the hue and the chroma saturation depending on the white balance.
Thus, the white balance control circuit 212 supplies white balance correction information to the hue correcting circuit 213. The hue correcting circuit 213 calculates an optimum hue from the white balance information and controls the linear matrix circuit 209 and the color difference gain control circuit 210 so as to obtain the optimum hue. Herein, the output signals of the linear matrix circuit 209 are given by:(R−Y)′=(R−Y)+α(B−Y) (−1<α<1)  (1)(B−Y)′=(B−Y)+β(R−Y) (−1<β1)  (2)Thus, it is possible to adjust the hue of the color difference signals by varying factors α and β, in equations (1) and (2), of the linear matrix circuit 209. That is, the hue correcting circuit 213 corrects the hue by adjusting a and β associated with the linear matrix circuit 209.
In the case where the flash apparatus 214 is used, it is required to correct the white balance depending on the intensity of ambient light and the amount of light generated by the flash apparatus 214. The flash control circuit 215 supplies information representing the intensity of ambient light determined from the luminance signal Y and also representing the amount of light generated by the flash apparatus 215 to the white balance control circuit 212. In accordance with the received information, the white balance control circuit 212 controls the white balance for an image taken using the flash apparatus 214.
In the above-described conventional image pickup apparatus, although the white balance can be corrected for an image taken using the flash apparatus, the hue or chroma saturation is not adjusted in accordance with the color rendering of the flash.
FIGS. 5A and 5B illustrate spectral distributions of solar light and flash light, respectively. As can be seen from FIGS. 5A and 5B, the color rendering of the flash is lower than that of the solar light. This causes not only a difference in color temperature but also a difference in color balance.
This means that when a picture is taken using a flash apparatus under conditions in terms of hues optimized for solar light, it is not always possible to achieve good color balance.